There is known a multibeam scanning optical apparatus having a plurality of individually modulated light sources and capable of scanning a plurality of lines at the same time. In such a multibeam scanning optical apparatus, an attempt is made to design a lens form of an optical element such as an f-theta (fθ) lens in an appropriate manner so that the size of the f-theta lens can be reduced and high-definition printing is realized. In this scanning optical apparatus, a curvature of f-theta lens in a sub-scanning plane changes asymmetrically along a main scanning direction from a point on an optical axis toward both outer ends of the f-theta lens.
However, if a shape of each lens surface of the optical element is not symmetrical with respect to the sub-scanning plane passing through the optical axis, fabrication and inspection of the optical element become complicated.
In view of the above, it would be desirable to provide a scanning optical apparatus which can be easily fabricated by designing the shape of each lens surface of the optical element to be symmetrical in the main scanning direction with respect to the optical axis, while enabling high-definition exposure of a photoconductor to light.